>How does this then imply that "adaptationist hypotheses are more readily >testable?"

Because when you hypothesize an evolved function for a trait, you can
generate predictions about what else must also be true if your hypothesis
is correct. Test those predictions. The answer will generate more
testable predictions. Test them.

You may be more creative than I, but with most of the traits we would
find interesting enough to study, I do not see how one would test non-
functionalist hypotheses without first falsifying functionalist alternatives.